1. Field of the Invention
The present invention relates to a hydraulic booster for use in a hydraulic braking system of an automotive vehicle, and more particularly to a hydraulic booster provided with a pressure control valve having a spool moved in response to movement of an input rod between a power position affording the communication between a boost chamber and a hydraulic power source, and a stop position affording the communication between the boost chamber and a reservoir, and having an axial hole affording the communication between the reservoir and the boost chamber in the stop position and at least two radial holes facing each other for introducing a hydraulic power pressure from the hydraulic power source into the axial hole of the spool in the power position.
2. Description of the Prior Art
In conventional service braking system for a vehicle, a boosting device called a servo or a booster is mounted in order to reduce a force applied on a brake pedal in braking operation, and compressed air, negative pressure in an intake manifold (for a vacuum booster) or fluid pressure (for a hydraulic booster) is used as a power source.
For example, as disclosed in Japanese Patent Laid-open Publication No. 59-209948, a hydraulic booster using the hydraulic pressure is so structured that a hydraulic pressure generator such as a master cylinder is boosted in response to depression of a brake pedal with a hydraulic power pressure discharged from a hydraulic power source. This hydraulic booster is disposed in a tandem master cylinder and includes a pressure control valve having a spool moved in response to movement of an input rod between a power position affording the communication between a boost chamber and the hydraulic power source, and a stop position affording the communication between the boost chamber and a reservoir. The spool has an axial hole affording the communication between the reservoir and the boost chamber in the stop position and two radial holes facing each other for introducing a hydraulic power pressure from the hydraulic power source into the axial hole through the radial holes in the power position.
However, in the conventional hydraulic booster as described above, since the radial holes are provided to face each other in order to prevent the spool from being applied with uneven load, when the spool is moved to its power position for blocking the communication between the boost chamber and the reservoir, and affording the communication between the boost chamber and the hydraulic power source in accordance with the displacement of the input rod moved in response to depression of the brake pedal, streams of the hydraulic power pressure fed from the hydraulic power source are introduced from the radial holes into the axial hole to interfere with each other. Thus, when the spool is abruptly moved, the opposing jet streams of the hydraulic power pressure will severely interfere with each other to cause an uncomfortable noise.